Carbon nanotube composite structure and method of manufacturing the same

ABSTRACT

Provided are a carbon nanotube structure more excellent in electric conductivity, thermal conductivity, and mechanical strength, and a method of manufacturing the carbon nanotube structure. A carbon nanotube composite structure is characterized by including: a first carbon nanotube structure in which functional groups bonded to plural carbon nanotubes are chemically bonded and mutually cross-linked to construct a network structure; and a second carbon nanotube structure in which functional groups bonded to plural carbon nanotubes are chemically bonded and mutually cross-linked to construct a network structure, the second carbon nanotube structure being combined with the network structure of the first carbon nanotube structure.

FIELD OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a carbon nanotube structure and amethod of manufacturing the same.

Carbon nanotubes (CNTs), with their unique shapes and characteristics,are being considered for various applications. A carbon nanotube has atubular shape of one-dimensional nature which is obtained by rolling oneor more graphene sheets composed of six-membered rings of carbon atomsinto a tube. A carbon nanotube which is formed from one graphene sheetis called a single-wall nanotube (SWNT) while a carbon nanotube which isformed from plural graphene sheets is called a multi-wall nanotube(MWNT). SWNTs are about 1 nm in diameter whereas multi-wall carbonnanotubes measure several tens nm in diameter, and both are far thinnerthan their predecessors, which are called carbon fibers.

One of the characteristics of carbon nanotubes resides in that theaspect ratio of length to diameter is very large since the length ofcarbon nanotubes is on the order of micrometers. Carbon nanotubes areunique in their extremely rare nature of being both metallic andsemiconductive because six-membered rings of carbon atoms in carbonnanotubes are arranged into a spiral. In addition, the electricconductivity of carbon nanotubes is very high and allows a current flowat a current density of 100 MA/cm² or more.

Carbon nanotubes excel not only in electrical characteristics but alsoin mechanical characteristics. That is, the carbon nanotubes aredistinctively tough, as attested by their Young's moduli exceeding 1TPa, which belies their extreme lightness resulting from being formedsolely of carbon atoms. In addition, the carbon nanotubes have highelasticity and resiliency resulting from their cage structure. Havingsuch various and excellent characteristics, carbon nanotubes are veryappealing as industrial materials.

Applied researches that exploit the excellent characteristics of carbonnanotubes have been heretofore made extensively. To give a few examples,a carbon nanotube is added as a resin reinforcer or as a conductivecarbon nanotube structure while another research utilizes a carbonnanotube as a probe of a scanning probe microscope. Carbon nanotubeshave also been utilized as minute electron sources, field emissionelectronic devices, and flat displays. An application that is beingdeveloped is to use a carbon nanotube as a hydrogen storage.

However, it is extremely difficult to actually arrange carbon nanotubes.Several techniques for wiring carbon nanotubes have been currentlyattempted.

A first technique includes: picking up one or several carbon nanotubesby using a manipulator in a scanning electron microscope; and arrangingthe one or several carbon nanotubes at a desired position. A techniquefor arranging carbon nanotubes by using a probe microscope may be givenas an example of a modification of the first technique. However, thetechnique requires much time and labor. Therefore, the technique issuitable for fundamental studies but is not practical.

A second technique is a technique for orienting a carbon nanotube in acertain direction by using electrophoresis. With this technique, carbonnanotubes may be wired in one direction, but it is difficult to wirecarbon nanotubes in multiple directions. Thus, this technique is notrealistic.

A third technique is a technique employing a chemical vapor deposition(CVD) method. The CVD method includes: using an acetylene gas or methanegas containing carbon as a raw material; and producing a carbon nanotubeby a chemical decomposition reaction of the raw material gas.

Cassell, N. Franklin, T. Tombler, E. Chan, J. Han, H. Dai, J. Am. Chem.Soc. 121, 7975-7976 (1999) discloses a method of wiring a carbonnanotube horizontally to a substrate. That is, disclosed is a techniqueincluding: fabricating an Si pillar on a substrate; mounting an additiveon the top part of the pillar; and allowing a methane gas to flow tobridge a carbon nanotube between pillars. The method by this techniquehas certainly enabled horizontal wiring. However, the probability ofcross-link is extremely low, and wiring at an arbitrary position isstill difficult.

As described above, a technique for wiring one or several carbonnanotubes is still at a developmental stage.

In the meantime, a method for wiring or patterning using a carbonnanotube as a film has been developed. For example, pattern formation ofa carbon nanotube has been heretofore performed by using a screenprinting method or a photolithography technique. Each of thosetechniques is excellent in forming a pattern in a wide area at once, andis used for patterning of an electron source in a field emission typedisplay (FED). However, in each of those methods, a carbon nanotube ismerely dispersed in a solvent before application, or is mixed with abinder before application. Therefore, the carbon nanotube isinsufficient in terms of performance such as a mechanical strength orelectric conductivity, and is hardly used directly as an electrode or anelectric circuit.

JP 2002-503204 A discloses that a carbon nanotube with athree-dimensional structure can be formed by using a functionalizedcarbon nanotube. However, this publication discloses that, for simpleuse in a chromatography-flow cell electrode, a product obtained bydepositing onto a metal mesh a carbon nanotube to which a functionalgroup that is porous and serves to separate and suck a passing substancehas been bonded is made porous, or carbon nanotubes are bonded to eachother by using an alkoxide of aluminum or silica (the alkoxide itselfserves as an insulator) as a cross-linking agent.

SUMMARY OF THE INVENTION

However, alkoxides cross-link each other, so that, in the resultantcarbon nanotube structure, alkoxide residues with several tens ofcross-linked sites randomly form a chain, and a distance between carbonnanotubes and a chemical structure of a carbon nanotube vary at eachcross-linked site. Therefore, it becomes difficult to obtain intendedcharacteristics of the resultant carbon nanotube structure, which placeslimitations on the use of the resultant carbon nanotube structure invarious applications. In addition, a network structure of carbonnanotubes is not formed sufficiently densely, so that there arises aproblem in that excellent characteristics including electricconductivity, thermal conductivity, and a mechanical strength which acarbon nanotube intrinsically has can not be sufficiently utilized.

The present invention has been made in view of the above circumstancesand provides a novel carbon nanotube structure excellent in electricconductivity, thermal conductivity, and mechanical strength, and amethod of manufacturing the carbon nanotube structure.

In order to solve the above problems, a carbon nanotube compositestructure according to the present invention is characterized byincluding: a first carbon nanotube structure in which functional groupsbonded to plural carbon nanotubes are chemically bonded and mutuallycross-linked to construct a network structure; and a second carbonnanotube structure in which functional groups bonded to plural carbonnanotubes are chemically bonded and mutually cross-linked to construct anetwork structure, the second carbon nanotube structure being combinedwith the network structure of the first carbon nanotube structure.

In the carbon nanotube composite structure according to the presentinvention, the second carbon nanotube structure is combined with thenetwork structure of the first carbon nanotube structure. Thecombination enables the second carbon nanotube structure to penetrateinto a gap between nanotubes in the network structure of the firstcarbon nanotube structure, and enables the density of a carbon nanotubestructure to increase.

As a result, a tougher carbon nanotube structure can be obtained, andthermal or electric (carrier) conductivity can be improved.

The composite structure in the present invention refers to a state wherethe first carbon nanotube structure and the second carbon nanotubestructure are integrated into one unit. The first carbon nanotubestructure and the second carbon nanotube structure may be complicatedlyentangled with each other to be integrated into one unit. Alternatively,carbon nanotubes constituting the first and second carbon nanotubestructures may cross-link each other.

Furthermore, in the carbon nanotube composite structure according to thepresent invention, if an average diameter of the carbon nanotubesconstituting the first carbon nanotube structure is different from anaverage diameter of the carbon nanotubes constituting the second carbonnanotube structure, densities of carbon nanotubes can be furtherincreased when the first carbon nanotube structure and the second carbonnanotube structure are combined with each other. Moreover, if the carbonnanotubes constituting the first carbon nanotube structure are formedmainly of multi-wall carbon nanotubes and the carbon nanotubesconstituting the second carbon nanotube structure are formed mainly ofsingle-wall carbon nanotubes, large network structure gaps formed in thefirst carbon nanotube structure are filled with the single-wall carbonnanotubes constituting the second carbon nanotube structure. As aresult, the density of the carbon nanotube composite structure can beincreased, and the mechanical strength and electric conductivity of thecarbon nanotube composite structure are expected to increase.

The first and/or second carbon nanotube structures are preferably formedby curing a solution containing plural carbon nanotubes to whichfunctional groups are bonded to thereby chemically bond together theplural functional groups bonded to the carbon nanotubes to form across-linked site.

Of those, a first structure preferable as the cross-linked site is astructure obtained by cross-linking together the plural functionalgroups with a cross-linking agent in the solution. More preferably, thecross-linking agent is not self-polymerizable.

If the carbon nanotube structure is formed by curing a solution asdescribed above, the cross-linked site in which the carbon nanotubescross-link each other can form a cross-linked structure in whichresidues of the functional groups remaining after the cross-linkingreaction are connected to each other with a connecting group that is aresidue remaining after the cross-linking reaction of the cross-linkingagent.

If the cross-linking agent has a property of polymerizing with othercross-linking agents (self-polymerizability), the connecting groupcontains a polymer in which two or more cross-linking agents areconnected to each other in some cases. In such cases, a substantialdensity of the carbon nanotubes in the carbon nanotube structuredecreases, and sufficient electric conductivity and mechanical strengthmay not be obtained.

On the other hand, if the cross-linking agent is not self-polymerizable,a gap between each of the carbon nanotubes can be controlled to a sizeof a cross-linking agent residue used. Therefore, a desired networkstructure of carbon nanotubes can be obtained with high duplicability.Further, reducing the size of the cross-linking agent residue canextremely narrow a gap between each of the carbon nanotubes bothelectrically and physically. In addition, carbon nanotubes in thestructure can be densely structured.

Therefore, if the cross-linking agent is not self-polymerizable, thecarbon nanotube structure of the present invention can exhibit inherentelectrical characteristics or mechanical characteristics of the carbonnanotubes in an extremely high level. In the present invention, the term“self-polymerizable” refers to a property of which the cross-linkingagents may prompt a polymerization reaction with each other in thepresence of other components such as water or in the absence of othercomponents. On the other hand, the term “not self-polymerizable” meansthat the cross-linking agent has no such a property.

If a cross-linking agent which is not self-polymerizable is selected asthe cross-linking agent, a cross-linked site, where carbon nanotubes inthe carbon nanotube composite structure of the present invention aremutually cross-linked, has primarily an identical cross-linkingstructure. Furthermore, the connecting group preferably employs ahydrocarbon as its skeleton, and the number of carbon atoms of theskeleton is preferably 2 to 10. Reducing the number of carbon atoms canshorten the length of a cross-linked site and sufficiently narrow a gapbetween carbon nanotubes as compared to the length of a carbon nanotubeitself. As a result, a carbon nanotube structure of a network structurecomposed substantially only of carbon nanotubes can be obtained.

Examples of the functional group include —OH, —COOH, —COOR (where Rrepresents a substituted or unsubstituted hydrocarbon group), —COX(where X represents a halogen atom), —NH₂, and —NCO. A selection of atleast one functional group selected from the group consisting of theabove functional groups is preferable, and in such a case, across-linking agent, which may prompt a cross-linking reaction with theselected functional group, is selected as the cross-linking agent.

Further, examples of the preferable cross-linking agent include apolyol, a polyamine, a polycarboxylic acid, a polycarboxylate, apolycarboxylic acid halide, a polycarbodiimide, and a polyisocyanate. Aselection of at least one cross-linking agent selected from the groupconsisting of the above cross-linking agents is preferable, and in sucha case, a functional group, which may prompt a cross-linking reactionwith the selected cross-linking agent, is selected as the functionalgroup.

At least one functional group and one cross-linking agent are preferablyselected respectively from the group exemplified as the preferablefunctional group and the group exemplified as the preferablecross-linking agent, so that a combination of the functional group andthe cross-linking agent may prompt a cross-linking reaction with eachother.

Examples of the particularly preferable functional group include —COOR(where R represents a substituted or unsubstituted hydrocarbon group).Introduction of a carboxyl group into carbon nanotubes is relativelyeasy, and the resultant substance (a carbon nanotube carboxylic acid) ishighly reactive. Therefore, after the formation of the substance, it isrelatively easy to esterify the substance to convert its functionalgroup into —COOR (where R represents a substituted or unsubstitutedhydrocarbon group), and such a functional group easily prompts across-linking reaction and is suitable for formation of a structure.

A polyol can be exemplified as the cross-linking agent corresponding tothe functional group. A polyol is cured by a reaction with —COOR (whereR represents a substituted or unsubstituted hydrocarbon group), andforms a robust cross-linked substance with ease. Among polyols, each ofglycerin and ethylene glycol reacts with the above functional groupswell. Moreover, each of glycerin and ethylene glycol itself has highbiodegradability, and applies a light load to an environment.

In the cross-linked site in which plural carbon nanotubes mutuallycross-link, the functional group is —COOR (where R represents asubstituted or unsubstituted hydrocarbon group). The cross-linked siteis —COO(CH₂)₂OCO— in the case where ethylene glycol is used as thecross-linking agent. In the case where glycerin is used as thecross-linking agent, the cross-linked site is —COOCH₂CHOHCH₂OCO— or—COOCH₂CH(OCO—)CH₂OH— if two OH groups contribute to the cross-linking,and the cross-linked site is —COOCH₂CH(OCO—)CH₂OCO— if three OH groupscontribute to the cross-linking. The chemical structure of thecross-linked site may be a chemical structure selected from the groupconsisting of the above four structures.

A second structure preferable as the structure of the cross-linked sitein the first and/or second carbon nanotube structures is a structureformed by chemical bonding of plural functional groups. More preferably,a reaction that causes the chemical bonding is any one of dehydrationcondensation, a substitution reaction, an addition reaction, and anoxidative reaction.

In the carbon nanotube structure, carbon nanotubes form a cross-linkedsite by chemically bonding together functional groups bonded to thecarbon nanotubes, to thereby form a network structure. Therefore, thesize of the cross-linked site for bonding the carbon nanotubes becomesconstant depending on the functional group to be bonded. Since a carbonnanotube has an extremely stable chemical structure, there is a lowpossibility that functional groups or the like excluding a functionalgroup to modify the carbon nanotube are bonded to the carbon nanotube.In the case where the functional groups are chemically bonded together,the designed structure of the cross-linked site can be obtained, and thecarbon nanotube structure can be homogeneous.

Furthermore, the functional groups are chemically bonded together, sothat the length of the cross-linked site between the carbon nanotubescan be shorter than that in the case where the functional groups arecross-linked with a cross-linking agent. Therefore, the carbon nanotubestructure is dense, and tends to readily produce an effect peculiar to acarbon nanotube.

In the carbon nanotube composite structure of the present invention,plural carbon nanotubes form a network structure via multiplecross-linked sites. As a result, excellent characteristics of a carbonnanotube can be stably utilized unlike a material such as a mere carbonnanotube dispersion film or a resin dispersion film in which carbonnanotubes accidentally contact each other and are substantially isolatedfrom each other.

The chemical bonding of plural functional groups is preferably oneselected from —COOCO—, —O—, —NHCO—, —COO—, and —NCH— in a condensationreaction. The chemical bonding is preferably at least one selected from—NH—, —S—, and —O— in a substitution reaction. The chemical bonding ispreferably —NHCOO— in an addition reaction. The chemical bonding ispreferably —S—S— in an oxidative reaction.

Examples of the functional group to be bonded to a carbon nanotube priorto the reaction include —OH—, —COOH, —COOR (where R represents asubstituted or unsubstituted hydrocarbon group), —X, —COX (where Xrepresents a halogen atom), —SH, —CHO, —OSO₂CH₃, —OSO₂(C₂H₅)CH₂—NH₂, and—NCO. It is preferable to select at least one functional group from thegroup consisting of the above functional groups.

Particularly preferable examples of the functional group include —COOH.A carboxyl group can be relatively easily introduced into a carbonnanotube. In addition, the resultant substance (a carbon nanotubecarboxylic acid) is highly reactive, easily causes a condensationreaction by using a dehydration condensation agent such asN-ethyl-N′-(3-dimethylaminopropyl)carbodiimide, and is thus suitable forforming a structure.

It should be noted that the structures of the cross-linked sites in thefirst carbon nanotube structure and the second carbon nanotube structuremay be identical to or different from each other. In addition, thenumber of carbon nanotube structures to be combined is not limited totwo, and has only to be two or more.

(Production Method)

Subsequently, a method of manufacturing a carbon nanotube compositestructure according to the present invention is characterized byincluding: a first supplying step of supplying the surface of asubstrate with a first solution containing plural carbon nanotubes towhich functional groups are bonded; a first cross-linking step ofchemically bonding the functional groups to form a first carbon nanotubestructure in which the plural carbon nanotubes mutually cross-link toconstruct a network structure; a second supplying step of supplying thefirst carbon nanotube structure with a second solution containing pluralcarbon nanotubes to which functional groups are bonded; and a secondcross-linking step of chemically bonding the functional groups in thesecond solution to form a second carbon nanotube structure in which theplural carbon nanotubes mutually cross-link to construct a networkstructure, the second carbon nanotube structure being combined with thefirst carbon nanotube structure.

In the present invention, first, in the step of supplying a substratewith a solution containing plural carbon nanotube having functionalgroups (hereinafter, referred to as “cross-linking solution” in somecases), the whole surface of the substrate or a part of the surface ofthe substrate is supplied with the solution. Then, in the subsequentcross-linking step, the solution after the supply is cured to form acarbon nanotube structure in which the plural carbon nanotubes mutuallycross-link via chemical bonding of the functional groups to construct anetwork structure. Passing those two steps can stabilize the structureitself of the carbon nanotube structure on the substrate, therebyresulting in the first carbon nanotube structure. Subsequently, insimilar steps, the first carbon nanotube structure is supplied with(immersed in) the second solution for cross-linking, thereby resultingin the second carbon nanotube structure combined with the first carbonnanotube structure.

Compositions of the first solution and the second solution arepreferably different from each other. In this case, a second structuredifferent from the first carbon nanotube structure is formed to resultin a high-density network structure. The compositions can be changed bychanging, for instance, diameters or layer structures of nanotubes to beused, carbon nanotube concentrations in the solutions, an average lengthof carbon nanotubes, or the types of carbon nanotubes. In addition, thecompositions can also be changed by a combination of those changes.

At this time, an average diameter of the carbon nanotubes in the firstsolution is preferably different from an average diameter of the carbonnanotubes in the second solution. In this case, the degree ofcombination of the first carbon nanotube structure and the second carbonnanotube structure can be improved to increase the density of the carbonnanotube composite structure. Furthermore, in this case, gaps in thefirst carbon nanotube structure that has been previously formed and hasa rough density can be bridged by thin carbon nanotubes, so that thedensity of the carbon nanotube composite structure is further increased.In addition, it is preferable that main carbon nanotubes in the firstsolution be multi-wall carbon nanotubes, and that main carbon nanotubesin the second solution be single-wall carbon nanotubes. In this case,large network structure gaps formed in the first carbon nanotubestructure are bridged by the single-wall carbon nanotubes constitutingthe second carbon nanotube structure. As a result, densities of carbonnanotubes can be increased, and the mechanical strengths and electricconductivities of the carbon nanotubes are expected to increase.

In forming chemical bonding between functional groups bonded to carbonnanotubes in the first solution and/or the second solution, a firstmethod preferable for forming a cross-linked site is a method ofcross-linking the functional groups with a cross-linking agent in thesolution. More preferably, the cross-linking agent is notself-polymerizable.

In the method of manufacturing a carbon nanotube composite structure ofthe present invention, examples of the functional group for forming thecross-linked site using the cross-linking agent include —OH, —COOH,—COOR (where R represents a substituted or unsubstituted hydrocarbongroup), —COX (where X represents is a halogen atom), —NH₂, and —NCO. Aselection of at least one functional group from the group consisting ofthe above functional groups is preferable, and in such a case, across-linking agent, which may prompt a cross-linking reaction with theselected functional group, is selected as the cross-linking agent.

Further, examples of the preferable cross-linking agent include apolyol, a polyamine, a polycarboxylic acid, a polycarboxylate, apolycarboxylic acid halide, a polycarbodiimide, and a polyisocyanate. Aselection of at least one cross-linking agent from the group consistingof the above cross-linking agents is preferable, and in such a case, afunctional group, which may prompt a cross-linking reaction with theselected cross-linking agent, is selected as the functional group.

At least one functional group and one cross-linking agent are preferablyselected respectively from the group exemplified as the preferablefunctional group and the group exemplified as the preferablecross-linking agent, so that a combination of the functional group andthe cross-linking agent may prompt a cross-linking reaction with eachother.

Particularly preferable examples of the functional group include —COOR(where R represents a substituted or unsubstituted hydrocarbon group). Acarboxyl group can be relatively easily introduced into a carbonnanotube, and the resultant substance (a carbon nanotube carboxylicacid) is highly reactive. Therefore, after the formation of thesubstance, it is relatively easy to esterify the substance to convertits functional group into —COOR (where R represents a substituted orunsubstituted hydrocarbon group). The functional group easily causes across-linking reaction, and is suitable for the formation of thestructure.

In addition, a polyol may be the cross-linking agent corresponding tothe functional group. A polyol is cured by a reaction with —COOR (whereR represents a substituted or unsubstituted hydrocarbon group), andforms a robust cross-linked substance with ease. Among polyols, each ofglycerin and ethylene glycol reacts with the above functional groupswell. Moreover, each of glycerin and ethylene glycol itself has highbiodegradability, and applies a light load to an environment.

Further, a second preferable method of forming a cross-linked site byusing the functional groups bonded to the carbon nanotubes in the firstand/or second solutions is a method of chemically bonding pluralfunctional groups together.

From the above, the size of the cross-linked site, which bonds thecarbon nanotubes together, becomes constant depending on the functionalgroup to be bonded. A carbon nanotube has an extremely stable chemicalstructure, so that a possibility of bonding of functional groups or thelike excluding the functional groups intended for a modification, islow. When chemically bonding the functional groups together, thedesigned structure of the cross-linked site can be obtained, providing ahomogeneous carbon nanotube structure.

Further, functional groups are chemically bonded together and thus thelength of the cross-linked site between the carbon nanotubes can beshortened compared to the case of cross-linking the functional groupstogether using a cross-linking agent. Therefore, the carbon nanotubestructure becomes dense, and effects peculiar to carbon nanotubes areeasily obtained.

Examples of a particularly preferable reaction, which chemically bondsthe functional groups together, include a condensation reaction, asubstitution reaction, an addition reaction, and an oxidative reaction.

In a method of manufacturing a carbon nanotube composite structure ofthe present invention, the preferable functional group includes: atleast one functional group selected from the group consisting of —COOR(where R represents a substituted or unsubstituted hydrocarbon group),—COOH, —COX (where X represents is a halogen atom), —OH, —CHO—, and —NH₂for the condensation reaction; at least one functional group selectedfrom the group consisting of —NH₂, —X (where X represents is a halogenatom), —SH, —OH, —OSO₂CH₃, and —OSO₂(C₆H₄)CH₃ for the substitutionreaction; at least one functional group selected from the groupconsisting of —OH and —NCO for the addition reaction; and —SH for theoxidative reaction.

In particular, in the method of manufacturing a carbon nanotubecomposite structure of the present invention, a molecule containing thefunctional groups may be bonded to carbon nanotubes to be chemicallybonded at the exemplified functional group portion to construct thecross-linked site.

If the reaction is dehydration condensation, a condensation agent ispreferably added. Further, the preferable functional group is at leastone functional group selected from the group consisting of —COOR (whereR represents a substituted or unsubstituted hydrocarbon group), —COOH,—COX (where X represents is a halogen atom), —OH, —CHO, and —NH₂.

For example, —COOH is particularly preferably used as the functionalgroup specifically used for the condensation reaction. Introduction of acarboxyl group into carbon nanotubes is relatively easy. Moreover, theresultant substance (a carbon nanotube carboxylic acid) is highlyreactive. Therefore, introduction of functional groups for forming anetwork structure into plural places of one carbon nanotube is easy. Inaddition, the functional group easily prompts in a condensationreaction, thus being suitable for the formation of the carbon nanotubestructure.

The resultant carbon nanotube structure can be combined with anothermaterial such as polymer, ceramics, or metal to form a composite.

A formation approach to a cross-linked site in forming a first carbonnanotube may be identical to or different from a formation approach to across-linked site in forming a second carbon nanotube. In other words,one approach may involve the use of a cross-linking agent to form across-linked site, and the other approach may involve bonding functionalgroups. Alternatively, both the approaches may involve the use of across-linking agent to combine different functional groups with thecross-linking agent. However, in order to increase the density of acarbon nanotube of a carbon nanotube structure in a composite structure,methods of forming a functional group and a cross-linked site arepreferably the same. This is because a functional group that has notbeen cross-linked in forming the first carbon nanotube structure can becross-linked with a functional group of a carbon nanotube in the secondsolution during the cross-linking step of the second carbon nanotubestructure, with the result that bonding between the first and secondcarbon nanotube structures proceeds.

The first carbon nanotube structure and/or the second carbon nanotubestructure to be used for a carbon nanotube composite structure accordingto the present invention may be a carbon nanotube structure layer havinga laminar structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiment of the present invention will be described indetail based on the following figures, wherein:

FIG. 1 is a schematic diagram showing a multi-wall nanotube-single-wallnanotube composite according to an embodiment of the present invention;

FIG. 2 is an enlarged schematic diagram of a part of the composite shownin FIG. 1;

FIG. 3 is a reaction scheme for the synthesis of a carbon nanotubecarboxylic acid in (Addition Step) of an example of the presentinvention;

FIG. 4 is a reaction scheme for esterification in (Addition Step) of theexample;

FIG. 5 is a reaction scheme for cross-linking by an ester exchangereaction in (Cross-linking Step) of the example; and

FIG. 6 is an electron micrograph of a multi-wall nanotube-single-wallnanotube composite formed in the example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, each of a carbon nanotube composite structure and a methodof manufacturing the carbon nanotube composite structure according tothe present invention will be described specifically through thedescription of the embodiments.

Carbon Nanotube Composite Structure

According to one aspect of the present invention, there is provided acarbon nanotube composite structure in which plural carbon nanotubestructures to be described in the section of <Carbon Nanotube Structure>are combined to increase densities of carbon nanotubes.

FIG. 1 is a schematic diagram of a carbon nanotube composite structure1. The carbon nanotube composite structure 1 is a composite of a firstcarbon nanotube structure 3 and a second carbon nanotube structure 4.The first carbon nanotube structure 3 is composed of plural multi-wallcarbon nanotubes 5. The second carbon nanotube structure 4 is formedinto a network in such a manner that gaps in the first carbon nanotubestructure are bridged by thinner single-wall nanotubes 6, and the secondcarbon nanotube structure 4 is also chemically bonded to the multi-wallcarbon nanotubes 5 constituting the first carbon nanotube structure.

FIG. 2 is a schematic diagram illustrating details of this structure.The plural multi-wall carbon nanotubes 5 constituting the first carbonnanotube structure 3 are cross-linked by first cross-linked sites 71.The plural single-wall carbon nanotubes 6 constituting the second carbonnanotube structure 4 are cross-linked by second cross-linked sites 72.Furthermore, the multi-wall carbon nanotubes 5 and the single-wallcarbon nanotubes 6 are cross-linked by third cross-linked sites 73 forcombination. In the figure, cross-linked sites are schematicallyenlarged. However, the cross-linked sites are actually the bondingbetween functional groups, and are sufficiently short as compared to thelength of a carbon nanotube. Therefore, carbon nanotubes are arrangedsuch that the carbon nanotubes are adjacently joined.

In the above example, multi-wall carbon nanotubes and single-wall carbonnanotubes have been used. However, carbon nanotubes constituting thefirst carbon nanotube structure may be identical to or different fromcarbon nanotubes constituting the second carbon nanotube structure.However, constituting one carbon nanotube structure by carbon nanotubesthinner than those constituting the other structure leads to an increasein filling density.

The structures of cross-linked sites may be different from or identicalto each other. Here, suppose that —COOH is used as a functional group,that a cross-linked site formation process is a cross-linking reactionusing glycerin or another reaction, and that a formation method for thefirst carbon nanotube structure is identical to a formation method forthe second carbon nanotube structure. In such a case, like the firstexample, —COOH that has not been cross-linked owing to the absence of—COOH as an object to be cross-linked in the vicinity thereof uponformation of the first carbon nanotube structure can be cross-linkedwith —COOH bonded to a carbon nanotube in the second solution used forforming the second carbon nanotube structure. As a result, the degree ofcombination of structures can be further improved.

Carbon Nanotube Structure

In the present invention, the phrase “carbon nanotube structure” refersto a member in which plural carbon nanotubes mutually cross-link toconstruct a network structure. Provided that a carbon nanotube structurecan be formed in such a manner that carbon nanotubes mutually cross-linkto construct a network structure, the carbon nanotube structure may beformed by any method. However, production according to [Method ofManufacturing Carbon Nanotube Composite Structure] to be described latercan be readily performed and can provide a high-performance carbonnanotube structure. Moreover, uniformization and control ofcharacteristics can be readily performed.

A basic structure for the first carbon nanotube structure is similar tothat for the second carbon nanotube structure. Unless a difference incharacteristic induced by the difference in basic structure is utilized,the first and second carbon nanotube structures will be describedwithout distinction.

A first structure for the carbon nanotube structure used as a carbonnanotube structure to be described later is manufactured by curing asolution (cross-linking solution) containing a carbon nanotube having afunctional group and a cross-linking agent that causes a cross-linkingreaction with the functional group to cause a cross-linking reactionbetween the functional group of the carbon nanotube and thecross-linking agent to thereby form a cross-linked site. Furthermore, asecond structure for the carbon nanotube structure is manufactured bychemically bonding functional groups of carbon nanotubes to formcross-linked sites.

Hereinafter, the carbon nanotube structure layer in the first carbonnanotube structure and/or the second carbon nanotube structure for usein a carbon nanotube composite structure according to the presentinvention will be described by way of examples of a method ofmanufacturing a carbon nanotube composite structure according to thepresent invention. Unless otherwise stated, the structures ofcross-linked sites are not considered.

(Carbon Nanotube)

Carbon nanotubes, which are the main component in the present invention,may be single-wall carbon nanotubes or multi-wall carbon nanotubes eachhaving two or more layers. Whether one or both types of carbon nanotubesare used (and, if only one type is to be used, which type is selected)may be decided appropriately taking into consideration the use of thecarbon nanotube structure or the cost.

Carbon nanotubes in the present invention include ones that are notexactly shaped like a tube, such as: a carbon nanohorn (a horn-shapedcarbon nanotube whose diameter continuously increases from one endtoward the other end) which is a variant of a single-wall carbonnanotube; a carbon nanocoil (a coil-shaped carbon nanotube forming aspiral when viewed in entirety); a carbon nanobead (a spherical beadmade of amorphous carbon or the like with its center pierced by a tube);a cup-stacked nanotube; and a carbon nanotube with its circumferencecovered with a carbon nanohorn or amorphous carbon.

Furthermore, carbon nanotubes in the present invention may be ones thatcontain some substances inside, such as: a metal-containing nanotubewhich is a carbon nanotube containing metal or the like; and a peapodnanotube which is a carbon nanotube containing a fullerene or ametal-containing fullerene. Also, the carbon nanotubes of either thesame type or different types may be used for the first structure and thesecond structure.

As described above, in the method of manufacturing a carbon nanotubecomposite structure of the present invention, it is possible to employcarbon nanotubes of any mode, including common carbon nanotubes,variants of common carbon nanotubes, and carbon nanotubes with variousmodifications, without a problem in terms of reactivity. Therefore, theconcept of “carbon nanotube” in the present invention encompasses all ofthe above.

Those carbon nanotubes are conventionally synthesized by a known method,such as arc discharge, laser ablation, and CVD, and the presentinvention can employ any of the methods. However, arc discharge in amagnetic field is preferable from the viewpoint of synthesizing a highlypure carbon nanotube.

Carbon nanotubes used in the present invention are preferably equal toand more than 0.3 nm and equal to or less than 100 nm in diameter. Ifthe diameter of the carbon nanotubes exceeds this upper limit, thesynthesis becomes difficult and costly. A more desirable upper limit ofthe diameter of the carbon nanotubes is 30 nm or less.

In general, the lower limit of the carbon nanotube diameter is about 0.3nm from a structural standpoint. However, too thin a diameter couldlower the synthesis yield. It is therefore preferable to set the lowerlimit of the carbon nanotube diameter to 1 nm or more, more preferably10 nm or more.

The length of carbon nanotubes used in the present invention ispreferably equal to or more than 0.1 μm and equal to or less than 100μm. If the length of the carbon nanotubes exceeds this upper limit, thesynthesis becomes difficult or requires a special method raising cost,which is not preferable. On the other hand, if the length of the carbonnanotubes falls short of this lower limit, the number of cross-linkbonding points per carbon nanotube is reduced, which is not preferable.A more preferable upper limit of the carbon nanotube length is 10 μm orless and a more preferable lower limit of the carbon nanotube length is1 μm or more.

The appropriate carbon nanotube content in the cross-linking solutionvaries depending on the length and thickness of carbon nanotubes,whether single-wall carbon nanotubes or multi-wall carbon nanotubes areused, the type and amount of functional groups in the carbon nanotubes,the type and amount of cross-linking agents or an additive for bondingfunctional groups together, the presence or absence of a solvent orother additive used and, if one is used, the type and amount of thesolvent or additive, etc. The carbon nanotube content in the solutionshould be high enough to form an excellent structure after applicationand curing but not too high to apply the liquid.

Specifically, the ratio of carbon nanotubes to the entire applicationliquid excluding the mass of the functional groups is 0.01 to 10 g/l,preferably 0.1 to 5 g/l, and more preferably 0.5 to 1.5 g/l, although,as mentioned above, the ranges could be different if the parameters aredifferent.

If the purity of carbon nanotubes to be used is not high enough, it isdesirable to raise the purity by refining the carbon nanotubes prior topreparation of the cross-linking solution. In the present invention, thehigher the carbon nanotube purity, the better the result can be.Specifically, the purity is preferably 90% or higher, more desirably,95% or higher. When the purity is low, cross-linking agents arecross-linked to carbon products such as amorphous carbon and tar, whichare impurities. This could change the cross-linking distance betweencarbon nanotubes, leading to a failure in obtaining desiredcharacteristics. No particular limitation is put on how carbon nanotubesare refined, and any known refining method can be employed.

(Functional Group 1)

In the first method in which the cross-linked site is formed using across-linking agent, carbon nanotubes can have any functional group tobe connected thereto, as long as functional groups selected can be addedto the carbon nanotubes chemically and can prompt a cross-linkingreaction with any type of cross-linking agent. Specific examples of suchfunctional groups include —COOR, —COX, —MgX, —X (where X representshalogen), —OR, —NR¹R², —NCO, —NCS, —COOH, —OH, —NH₂, —SH, —SO₃H,—R′CHOH, —CHO, —CN, —COSH, —SR, —SiR₁₃ (where R, R¹, R², and R′ eachrepresent a substituted or unsubstituted hydrocarbon group). Note thatemployable functional groups are not limited to those examples.

Of those, a selection of at least one functional group from the groupconsisting of —OH, —COOH, —COOR (where R represents a substituted orunsubstituted hydrocarbon group), —COX (where X represents is a halogenatom), —NH₂, and —NCO is preferable. In that case, a cross-linkingagent, which can prompt a cross-linking reaction with the selectedfunctional group, is selected as the cross-linking agent.

In particular, —COOR (where R represents a substituted or unsubstitutedhydrocarbon group) is particularly preferable. This is because acarboxyl group can be relatively easily introduced into a carbonnanotube, because the resultant substance (a carbon nanotube carboxylicacid) can be easily introduced as a functional group by esterifying thesubstance, and because the substance has good reactivity with across-linking agent.

R in the functional group —COOR is a substituted or unsubstitutedhydrocarbon group, and is not particularly limited. However, R ispreferably an alkyl group having 1 to 10 carbon atoms, more preferablyan alkyl group having 1 to 5 carbon atoms, and particularly preferably amethyl group or an ethyl group in terms of reactivity, solubility,viscosity, and ease of use as a solvent of a paint.

The appropriate amount of functional groups introduced varies dependingon the length and thickness of carbon nanotubes, whether single-wallcarbon nanotubes or multi-wall carbon nanotubes are used, the types offunctional groups, the use of the carbon nanotube structure, etc. Fromthe viewpoint of the strength of the cross-linked substance obtained,namely, the strength of the structure, a preferable amount of functionalgroups introduced is large enough to add two or more functional groupsto each carbon nanotube. How functional groups are introduced intocarbon nanotubes will be explained in the section below titled [Methodof Manufacturing a Carbon Nanotube Structure].

Any cross-linking agent, which is an essential ingredient of thecross-linking solution, can be used as long as the cross-linking agentis capable of prompting a cross-linking reaction with the functionalgroups of the carbon nanotubes. In other words, the types ofcross-linking agents that can be selected are limited to a certaindegree by the types of the functional groups. Also, the conditions ofcuring (heating, UV irradiation, irradiation of visible light, naturalcuring, etc.) as a result of the cross-linking reaction are naturallydetermined by the combination of those parameters.

Specific examples of the preferable cross-linking agents include apolyol, a polyamine, a polycarboxylic acid, a polycarboxylate, apolyearboxylic acid halide, a polycarbodiimide, and a polyisocyanate. Itis desirable to select at least one cross-linking agent from the groupconsisting of the above. In that case, a functional group which canprompt a reaction with the cross-linking agent is selected as thefunctional group.

At least one functional group and one cross-linking agent areparticularly preferably selected respectively from the group exemplifiedas the preferable functional group and the group exemplified as thepreferable cross-linking agent, so that a combination of the functionalgroup and the cross-linking agent may prompt a cross-linking reactionwith each other. The following Table 1 lists the combinations of thefunctional group of the carbon nanotubes and the correspondingcross-linking agent, which can prompt a cross-linking reaction, alongwith curing conditions of the combinations. TABLE 1 Functional group ofcarbon nanotube Cross-linking agent Curing condition —COOR Polyol heatcuring —COX Polyol heat curing —COOH Polyamine heat curing —COXPolyamine heat curing —OH Polycarboxylate heat curing —OH Polycarboxylicacid halide heat curing —NH₂ Polycarboxylic acid heat curing —NH₂Polycarboxylic acid halide heat curing —COOH Polycarbodiimide heatcuring —OH Polycarbodiimide heat curing —NH₂ Polycarbodiimide heatcuring —NCO Polyol heat curing —OH Polyisocyanate heat curing —COOHAmmonium complex heat curing —COOH cis-platin heat curing*where R represents a substituted or unsubstituted hydrocarbon group*where X represents a halogen

Of those combinations, preferable is the combination of —COOR (where Rrepresents a substituted or unsubstituted hydrocarbon group) with goodreactivity on a functional group side and a polyol, a polyamine, anammonium complex, congo red, and cis-platin, which form a robustcross-linked substance with ease. The terms “polyol”, “polyamine”, and“ammonium complex”, in the present invention are genetic names fororganic compounds each having two or more OH groups, NH₂ groups, andammonium groups, respectively. Of those, one having 2 to 10 (morepreferably 2 to 5) carbon atoms and 2 to 22 (more preferably 2 to 5) OHgroups is preferable in terms of cross-linkability, solventcompatibility when an excessive amount thereof is charged,processability of waste liquid after a reaction by virtue ofbiodegradability (environment aptitude), yield of polyol synthesis, andso on. In particular, the number of carbon atoms is preferably lowerwithin the above range because a gap between carbon nanotubes in theresultant structure can be narrowed to bring the carbon nanotubes intosubstantial contact with each other (to bring the carbon nanotubes closeto each other). Specifically, glycerin and ethylene glycol areparticularly preferable, and it is preferable to use one or both ofglycerin and ethylene glycol as a cross-linking agent.

From another perspective, the cross-linking agent is preferably anot-self-polymerizable cross-linking agent. In addition to glycerin andethylene glycol as examples of the polyols, butenediol, hexynediol,hydroquinone, and naphthalenediol are obviously not-self-polymerizablecross-linking agents. More generally, a prerequisite of thenot-self-polymerizable cross-linking agent is to be without a pair offunctional groups, which can prompt a polymerization reaction with eachother, in itself. On the other hand, examples of a self-polymerizablecross-linking agent include one that has a pair of functional groups,which can prompt a polymerization reaction with each other (alkoxide,for example).

(Functional Group 2)

Further, in the second method of obtaining a network structure ofmutually cross-linked carbon nanotubes, a cross-linked site of thecarbon nanotube structure is formed by chemically bonding pluralfunctional groups, in which at least one end of the cross-linked site isbonded to different carbon nanotubes respectively. In the second method,a functional group to be bonded to the carbon nanotubes is notparticularly limited as long as the functional group can be chemicallyadded to the carbon nanotubes and is capable of reacting to each otherwith any type of additive, and any functional group can be selected.Specific examples of the functional group include —COOR, —COX, —MgX—, —X(where X represents a halogen), —OR, —NR¹R², —NCO, —NCS, —COOH, —OH,—NH₂, —SH, —SO₃H, —R′CHOH, —CHO, —CN, —COSH, —SR, —SiR₁₃ (where R, R¹,R², and R³ each represent a substituted or unsubstituted hydrocarbongroup), but are not limited to those.

Of those, the preferable functional group includes: at least onefunctional group selected from the group consisting of —COOR (where Rrepresents a substituted or unsubstituted hydrocarbon group), —COOH,—COX (where X represents a halogen atom), —OH, —CHO—, and —NH₂ for thecondensation reaction; at least one functional group selected from thegroup consisting of —NH₂, —X (where X represents a halogen atom), —SH,—OH, —OSO₂CH₃, and —OSO₂(C₆H₄)CH₃ for the substitution reaction; atleast one functional group selected from the group consisting of —OH and—NCO for the addition reaction; and —SH for the oxidative reaction.

Further, it is also possible to bond a molecule, which partiallycontains those functional groups, with the carbon nanotubes to bechemically bonded at a preferable functional group portion exemplifiedabove. Even in this case, a functional group with a large molecularweight to be bonded to the carbon nanotubes is bonded as intended,enabling control of a length of the cross-linked site.

(Additive)

Any additive that is capable of making the functional groups of thecarbon nanotubes react to each other can be mixed in the cross-linkingsolution. In other words, the types of additives that can be selectedare limited to a certain degree by the types of the functional groupsand the reaction type. Also, the condition of curing (heating, UVirradiation, irradiation of visible light, natural curing, etc.) as aresult of the reaction is naturally determined by the combination ofthose parameters.

(Condensation Agent)

To give specific examples of preferable additives, an acid catalyst or adehydration condensation agent, for example, sulfuric acid,N-ethyl-N′-(3-dimethylaminopropyl)carbodiimide, and dicyclohexylcarbodiimide, is preferred as a condensation agent. Preferably, at leastone condensation agent is selected from the group consisting of theabove. The functional groups selected have to react to each other withthe help of the selected condensation agent.

(Base)

When a substitution reaction is to be utilized, a base is an essentialingredient of the cross-linking solution. An arbitrary base is selectedin accordance with the degree of acidity of hydroxyl groups.

Preferably, the base is at least one selected from the group consistingof sodium hydroxide, potassium hydroxide, pyridine, and sodium ethoxide.In that case, a substitution reaction is to take place among thefunctional groups with the help of the selected base.

It is particularly preferable to select a combination of functionalgroups such that at least two functional groups from each of the examplegroups that are given above as examples of preferable functional groupsreact to each other. Table 2 below lists functional groups of carbonnanotubes and names of the corresponding reactions. An addition reactiondoes not necessarily need an additive. In an oxidative reaction, anadditive is not necessarily needed but adding an oxidative reactionaccelerator is preferable. A specific example of the accelerator isiodine. TABLE 2 Functional group Functional group of carbon of carbonBonding site nanotube (A) nanotube (B) Reaction —COOCO— —COOH —Dehydration condensation —S—S— —SH — Oxidative reaction —O— —OH —Dehydration condensation —NH—CO— —COOH —NH₂ Dehydration condensation—COO— —COOH —OH Dehydration condensation —COO— —COOR —OH Dehydrationcondensation —COO— —COX —OH Dehydration condensation —CH═N— —CHO —NH₂Dehydration condensation —NH— —NH₂ —X Substitution reaction —S— —SH —XSubstitution reaction —O— —OH —X Substitution reaction —O— —OH —OSO₂CH₃Substitution reaction —O— —OH —OSO₂(C₆H₄)CH₃ Substitution reaction—NH—COO— —OH —N═C═O Addition reaction*where R represents a substituted or unsubstituted hydrocarbon group*where X represents a halogen

Next, the content of a cross-linking agent or of an additive for bondinga functional group in the cross-linking solution varies depending on thetype of the cross-linking agent (including whether the cross-linkingagent is self-polymerizable or not self-polymerizable) and the type ofthe additive for bonding a functional group. The content also variesdepending on the length and thickness of a carbon nanotube, whether thecarbon nanotube is of a single-wall type or a multi-wall type, the typeand amount of a functional group of the carbon nanotube, the presence orabsence, types, and amounts of a solvent and other additives, and thelike. Therefore, the content can not be determined uniquely. Inparticular, for example, glycerin or ethylene glycol can also providecharacteristics of a solvent because a viscosity of glycerin or ethyleneglycol is not so high, and thus an excessive amount of glycerin orethylene glycol can be added.

(Other Additive)

The cross-linking solution may contain various additives including asolvent, a viscosity modifier, a dispersant, and a cross-linkingaccelerator. A solvent is added when satisfactory application of thecross-linking solution is not achieved with solely the cross-linkingagents or the additive for bonding the functional groups. A solvent thatcan be employed is not particularly limited, and may be appropriatelyselected according to the types of the cross-linking agents. Specificexamples of employable solvents include: organic solvents such asmethanol, ethanol, isopropanol, n-propanol, butanol, methyl ethylketone, toluene, benzene, acetone, chloroform, methylene chloride,acetonitrile, diethyl ether, and tetrahydrofuran (THF); water; aqueoussolutions of acids; and alkaline aqueous solutions. A solvent as such isadded in an amount that is not particularly limited but determinedappropriately by taking into consideration the ease of applying thecross-linking solution.

A viscosity modifier is added when satisfactory application of thecross-linking solution is not achieved with solely the cross-linkingagents and the additive for bonding the functional groups. A viscositymodifier that can be employed is not particularly limited, and may beappropriately selected according to the cross-linking agents used.Specific examples of employable viscosity modifiers include methanol,ethanol, isopropanol, n-propanol, butanol, methyl ethyl ketone, toluene,benzene, acetone, chloroform, methylene chloride, acetonitrile, diethylether, and THF.

Some of those viscosity modifiers have the function of a solvent whenadded in a certain amount, and it is meaningless to apparentlydiscriminate viscosity modifiers from solvents. A viscosity modifier assuch is added in an amount that is not particularly limited butdetermined by taking into consideration the ease of applying thecross-linking solution.

A dispersant is added to the cross-linking solution in order to maintainthe dispersion stability of the carbon nanotubes, the cross-linkingagents, or the additive for bonding the functional groups in theapplication liquid. Various known surface-active agents, water-solubleorganic solvents, water, aqueous solutions of acids, alkaline aqueoussolutions, etc. can be employed as a dispersant. However, a dispersantis not always necessary since components of the coating material of thepresent invention have high dispersion stability by themselves. Inaddition, depending on the use of the structure after the formation, thepresence of a dispersant and like other impurities in the structure maynot be desirable. In such case, a dispersant is not added at all, or isadded in a very small amount.

(Method of Preparing the Cross-linking Solution)

A method of preparing a cross-linking solution is described next. Thecross-linking solution is prepared by mixing, as needed, carbonnanotubes that have functional groups with a cross-linking agent thatprompts a cross-linking reaction with the functional groups or anadditive that causes the functional groups to form chemical bondingamong themselves (mixing step). The mixing step may be preceded by anaddition step in which the functional groups are introduced into thecarbon nanotubes.

If carbon nanotubes having functional groups are starting materials, thepreparation starts with the mixing step. If normal carbon nanotubesthemselves are starting materials, the preparation starts with theaddition step. The addition step is a step of introducing desiredfunctional groups into carbon nanotubes. How functional groups areintroduced varies depending on the type of functional group. One methodis to add a desired functional group directly, and another method is tointroduce a functional group that is easy to attach and then substitutethe whole functional group or a part thereof or attach a differentfunctional group to the former functional group in order to obtain theobjective functional group. Still another method is to apply amechanochemical force to a carbon nanotube to break or modify only asmall portion of a graphene sheet on the surface of the carbon nanotubeand introduce various functional groups from the broken or modifiedportion.

Cup-stacked carbon nanotubes, which have many defects on the surfaceupon manufacture, and carbon nanotubes that are formed by vapor phasegrowth are relatively easy to introduce functional groups. On the otherhand, carbon nanotubes each having a perfect graphene sheet structureexert the carbon nanotube characteristics more effectively and areeasier to control the characteristics. Consequently, it is particularlypreferable to use a multi-wall carbon nanotube so that defects areformed as many as appropriate as a carbon nanotube structure on itsoutermost layer to bond functional groups for cross-linking while theinner layers having less structural defects exert the carbon nanotubecharacteristics.

There is no particular limitation put on the addition step and any knownmethod can be employed. Various addition methods disclosed in JP2002-503204 A may be employed in the present invention depending on thepurpose. A description is given on a method of introducing —COOR (whereR represents a substituted or unsubstituted hydrocarbon group), aparticularly desirable functional group among the functional groupslisted in the above. To introduce —COOR (where R represents asubstituted or unsubstituted hydrocarbon group) into carbon nanotubes,carboxyl groups may be (a) added to the carbon nanotubes once, and then(b) esterified.

(a) Addition of Carboxyl Group

To introduce carboxyl groups into carbon nanotubes, carboxyl groups arerefluxed together with an acid having an oxidizing effect. Thisoperation is relatively easy and is preferable since carboxyl groupswith high reactivity are attached to carbon nanotubes. A briefdescription of the operation is given below.

An acid having an oxidizing effect is, for example, concentrated nitricacid, hydrogen peroxide water, a mixture of sulfuric acid and nitricacid, or aqua regia. When concentrated nitric acid is used, inparticular, the concentration is preferably 5 mass % or higher, morepreferably, 60 mass % or higher.

A normal reflux method can be employed. The temperature at which refluxis performed is preferably set to a level near the boiling point of theacid used. When concentrated nitric acid is used, for instance, thetemperature is preferably set to 120 to 130° C. The reflux preferablylasts 30 minutes to 20 hours, more preferably, 1 hour to 8 hours.

Carbon nanotubes to which carboxyl groups are attached (a carbonnanotube carboxylic acid) are generated in the reaction liquid after thereflux. The reaction liquid is cooled down to room temperature and thenreceives a separation operation or washing as necessary, therebyobtaining the objective carbon nanotube carboxylic acid.

(b) Esterification

The target functional group —COOR (where R represents a substituted orunsubstituted hydrocarbon group) can be introduced by adding an alcoholto the obtained carbon nanotube carboxylic acid and dehydrating themixture for esterification.

The alcohol used for the esterification is determined according to R inthe formula of the functional group. That is, if R is CH₃, the alcoholis methanol, and if R is C₂H₅, the alcohol is ethanol. A catalyst isgenerally used in the esterification, and a conventionally knowncatalyst such as sulfuric acid, hydrochloric acid, or toluenesulfonicacid can also be used in the present invention. The use of sulfuric acidas a catalyst is preferable from a view of not prompting a side reactionin the present invention.

The esterification may be conducted by adding an alcohol and a catalystto a carbon nanotube carboxylic acid and refluxing the mixture at anappropriate temperature for an appropriate time period. A temperaturecondition and a time period condition in this case depend on type of acatalyst, type of alcohol, or the like and cannot be simply determined,but a reflux temperature close to the boiling point of the alcohol usedis preferable. The reflux temperature is preferably in the range of 60to 70° C. for methanol, for example. Further, a reflux time period ispreferably in the range of 1 to 20 hours, more preferably in the rangeof 4 to 6 hours.

A carbon nanotube with the functional group —COOR (where R represents asubstituted or unsubstituted hydrocarbon group) added can be obtained byseparating a reaction product from a reaction solution afteresterification and washing the reaction product as required.

The mixing step is a step of mixing, as required, carbon nanotubes whichcontain functional groups with a cross-linking agent prompting across-linking reaction with the functional groups or an additive forbonding the functional groups to prepare the cross-linking solution. Inthe mixing step, other components described in the aforementionedsection titled [Carbon Nanotube Structure] are mixed, in addition to thecarbon nanotubes containing functional groups and the cross-linkingagents. Then, preferably, an amount of a solvent or a viscosity modifieris adjusted considering ease of applying to prepare the cross-linkingsolution just before supply to the substrate.

Simple stirring with a spatula and stirring with a stirrer of a stirringblade type, a magnetic stirrer, and a stirring pump may be used.However, to achieve higher degree of uniformity in dispersion of thecarbon nanotubes to enhance storage stability while fully extending anetwork structure by cross-linking of the carbon nanotubes, anultrasonic disperser or a homogenizer may be used for powerfuldispersion. However, when using a stirring device with a strong shearforce of stirring such as a homogenizer, there arises a risk of cuttingand damaging the carbon nanotubes in the solution, thus the device maybe used for a very short time period.

A carbon nanotube composite structure is formed by supplying a substratesurface with the cross-linking solution described above and curing thesubstrate. A supplying method and a curing method are described indetail in the section below titled [Method of Manufacturing a CarbonNanotube Composite Structure].

The carbon nanotube structure in the present invention is in a statewhere carbon nanotubes are networked. In detail, the carbon nanotubestructure is cured into a matrix shape, carbon nanotubes are connectedto each other via cross-linked sites, and characteristics of a carbonnanotube itself such as high electron- and hole-transmissioncharacteristics can be exerted sufficiently. In other words, the carbonnanotube structure has carbon nanotubes that are tightly connected toeach other, contains no other binders and the like, and is thus composedsubstantially only of carbon nanotubes, so that characteristics peculiarto a carbon nanotube are fully utilized. The plural carbon nanotubestructures compose the carbon nanotube composite structure of thepresent invention. Thus, by increasing the density of the carbonnanotubes, the carbon nanotube composite structure can be utilized in awider range.

A thickness of the carbon nanotube structure of the present inventioncan be widely selected from being very thin to being thick according toan application. Lowering a content of the carbon nanotubes in thecross-linking solution used (simply, lowering the viscosity by diluting)and applying the cross-linking solution in a thin coat form allows avery thin coat to be obtained. Similarly, raising a content of thecarbon nanotubes allows a thick structure to be obtained. Further,repeating the application allows an even thicker structure to beobtained. Formation of a very thin structure from a thickness of about10 nm is possible, and formation of a thick structure without an upperlimit is possible through recoating. A possible coat thickness with onecoating is about 5 μm. Further, a desired shape of the structure can beobtained by injecting the cross-linking solution, in which a content orthe like is adjusted, to a mold and bonding.

In the first and/or second carbon nanotube structures, when using thecross-linking agent of the first method, a site where the carbonnanotubes cross-link together, that is, the cross-linked site formed bya cross-linking reaction between the functional groups of the carbonnanotubes and the cross-linking agents has a cross-linking structure. Inthe cross-linking structure, residues of the functional group remainingafter a cross-linking reaction are connected together with a connectinggroup, which is a residue of the cross-linking agent remaining after across-linking reaction.

As described, the cross-linking agent, which is a component of thecross-linking solution, is preferably not self-polymerizable. If thecross-linking agent is not self-polymerizable, the carbon nanotubestructure layer finally manufactured would be constructed from a residueof only one cross-linking agent. The gap between the carbon nanotubes tobe cross-linked can be controlled to a size of a residue of thecross-linking agent used, thereby providing a desired network structureof the carbon nanotubes with high duplicability. Further, pluralcross-linking agents are not present between the carbon nanotubes, thusenabling an enhancement of a substantial density of the carbon nanotubesin the carbon nanotube structure. Further, reducing a size of a residueof the cross-linking agent can extremely narrow a gap between each ofthe carbon nanotubes both electrically and physically (carbon nanotubesare substantially in direct contact with each other).

When forming the carbon nanotube structure with a cross-linking solutionprepared by selecting a single functional group of the carbon nanotubesand a single not-self-polymerizable cross-linking agent, thecross-linked site of the layer will have the same cross-linkingstructure (Example 1). Further, even when forming the carbon nanotubestructure layer with a cross-linking solution prepared by selectingplural types of functional groups of the carbon nanotubes and/or pluraltypes of not-self-polymerizable cross-linking agents, the cross-linkedsite of the layer will mainly have a cross-linking structure based on acombination of the functional group and the not-self-polymerizablecross-linking agent mainly used (Example 2).

On the contrary, when forming the carbon nanotube structure layer with across-linking solution prepared by selecting self-polymerizablecross-linking agents, without regard to whether the functional groupsand the cross-linking agents are of single or plural types, thecross-linked site of the layer where carbon nanotubes cross-linktogether will not mainly have a specific cross-linking structure. Thisis because the cross-linked site will be in a state where numerousconnecting groups with different connecting (polymerization) numbers ofthe cross-linking agents coexist.

In other words, by selecting not-self-polymerizable cross-linkingagents, the cross-linked sites, where the carbon nanotubes of the carbonnanotube structure layer cross-link together, bond with the functionalgroup through a residue of only one cross-linking agent, thus forming amainly identical cross-linking structure. “Mainly identical” here is aconcept including a case with all of the cross-linked sites having anidentical cross-linking structure as described above (Example 1), aswell as a case with the cross-linking structure based on a combinationof the functional group and the not-self-polymerizable cross-linkingagent mainly used becomes a main structure with respect to the wholecross-linked site as described above (Example 2).

When referring as “mainly identical”, a “ratio of identical cross-linkedsites” with respect to the whole cross-linked sites will not have auniform lower limit defined. The reason is that a case of imparting afunctional group or a cross-linking structure with an aim different fromformation of a carbon nanotube network may be assumed for example.However, in order to actualize high electrical or physicalcharacteristics peculiar to carbon nanotubes with a strong network, a“ratio of identical cross-linked sites” with respect to the totalcross-linked sites is preferably 50% or more, more preferably 70% ormore, further more preferably 90% or more, and most preferably 100%,based on numbers. Those number ratios can be determined through, forexample, a method of measuring an intensity ratio of an absorptionspectrum corresponding to the cross-linking structure with an infraredspectrum.

As described, if a carbon nanotube structure layer has the cross-linkedsite with a mainly identical cross-linking structure where carbonnanotubes cross-link, a uniform network of the carbon nanotubes can beformed in a desired state. In addition, the carbon nanotube network canbe constructed with homogeneous, satisfactory, and expected electricalor physical characteristics and high duplicability.

Further, the connecting group preferably contains hydrocarbon for askeleton thereof. “Hydrocarbon for a skeleton” here refers to a mainchain portion of the connecting group consisting of hydrocarbon, themain portion of the connecting group contributing to connecting residuestogether of the functional groups of carbon nanotubes to be cross-linkedremaining after a cross-linking reaction. A side chain portion, wherehydrogen of the main chain portion is substituted by anothersubstituent, is not considered. Obviously, it is more preferable thatthe whole connecting group consists of hydrocarbon.

The number of carbon atoms in the hydrocarbon is preferably 2 to 10,more preferably 2 to 5, and further more preferably 2 to 3. Theconnecting group is not particularly limited as long as the connectinggroup is divalent or more.

In the cross-linking reaction of the functional group —COOR (where Rrepresents a substituted or unsubstituted hydrocarbon) and ethyleneglycol, exemplified as a preferable combination of the functional groupof carbon nanotubes and the cross-linking agent, the cross-linked site,where plural carbon nanotubes cross-link to each other, becomes—COO(CH₂)₂OCO—.

Further, in the cross-linking reaction of the functional group —COOR(where R represents a substituted or unsubstituted hydrocarbon) andglycerin, the cross-linked site, where plural carbon nanotubescross-link to each other, becomes —COOCH₂CHOHCH₂OCO— and/or—COOCH₂CH(OCO—)CH₂OH if two OH groups contribute in the cross-link, andthe cross-linked site becomes —COOCH₂CH(OCO—)CH₂OCO— if three OH groupscontribute in the cross-link.

As has been described, the carbon nanotube structure has a networkstructure that is composed of plural carbon nanotubes connected to eachother through plural cross-linked sites. Thus, contact or arrangement ofcarbon nanotubes is not disturbed, unlike a mere carbon nanotubedispersion film. Therefore, there are stably obtained characteristicsthat are unique of carbon nanotubes, including: electricalcharacteristics such as high electron- and hole-transmissioncharacteristics; physical characteristics such as thermal conductivityand toughness; and light absorption characteristics.

Further, in the second method of forming the cross-linked site throughchemically bonding plural functional groups, in which at least one endof the cross-linked site of the first and/or second carbon nanotubestructures is bonded to different carbon nanotubes respectively, thecarbon nanotube structure has carbon nanotubes connected in a matrixform through a cross-linked portion. Therefore, characteristics ofcarbon nanotubes, such as high electron- and hole-transmissioncharacteristics, are easily obtained. That is, the carbon nanotubestructure has carbon nanotubes that are tightly connected together, andcontains no other binders. Therefore, the carbon nanotube structure canbe composed substantially only of carbon nanotubes.

Further, the cross-linked sites are formed by a reaction among thefunctional groups, thus enabling an enhancement of the actual carbonnanotube density of the carbon nanotube structure. If the functionalgroups are reduced in size, the carbon nanotubes can be brought veryclose to each other both electrically and physically, andcharacteristics of a carbon nanotube itself can be more easily obtained.

Further, cross-linked sites are chemical bonding of the functionalgroups, thus the carbon nanotube structures mainly have the samecross-linking structure. Therefore, a uniform network of carbonnanotubes can be formed into a desired state. Therefore, electrical andphysical carbon nanotube characteristics that are homogeneous andexcellent can be obtained. Furthermore, electrical or physicalcharacteristics expected from carbon nanotubes, or close to the expectedlevel or with high duplicability, can be obtained.

A layer except the carbon nanotube structure layer may be formed in thecarbon nanotube composite structure of the present invention. Forexample, placing an adhesive layer between the surface of the substrateand the carbon nanotube structure layer for enhancing adhesivenesstherebetween can improve the adhesive strength of a patterned carbonnanotube structure layer, and is thus preferable. In addition, theperiphery of the carbon nanotube structure can be coated with aninsulator, an electric conductor, or the like according to wireapplications.

Specifics of the above-described carbon nanotube composite structure ofthe present invention including its shape will be made clear in thefollowing section of [Method of Manufacturing a Carbon NanotubeComposite Structure] and Example. Note that the descriptions below showmerely examples and are not to limit specific modes of the carbonnanotube composite structure of the present invention.

[Method of Manufacturing a Carbon Nanotube Composite Structure]

A method of manufacturing a carbon nanotube composite structure of thepresent invention is a method suitable for manufacture of theabove-described carbon nanotube composite structure of the presentinvention. Specifically, the method of manufacturing a carbon nanotubecomposite structure of the present invention includes: (A-1) a firstsupplying step of supplying a surface of a substrate with a firstsolution that contains plural carbon nanotubes having functional groupsconnected thereto; (B-1) a first cross-linking step for forming a firstcarbon nanotube structure that has a network structure composed of theplural carbon nanotubes that are cross-linked to each other by chemicalbonding formed among the functional groups; (A-2) a second supplyingstep of supplying the first carbon nanotube structure with a secondsolution that contains plural carbon nanotubes having functional groupsconnected thereto; and (B-2) a second cross-linking step for forming asecond carbon nanotube structure that has a network structure composedof the plural carbon nanotubes that are cross-linked to each other bychemical bonding formed among the functional groups of the secondsolution, the second carbon nanotube structure being combined with thefirst carbon nanotube structure.

Hereinafter, an example of a method of manufacturing a carbon nanotubecomposite structure according to the present invention will be describedfor each step.

(A-1) First Supplying Step

In the present invention, the “supplying step” is a step of supplyingthe surface of a substrate such as a slide glass with a solutioncontaining a carbon nanotube having a functional group.

The supplying method is not particularly limited, and any method can beadopted from a wide range to supply the cross-linking applicationliquid. For example the liquid may be simply dropped or spread with asqueegee or may be applied by a common application method. Examples ofcommon application methods include spin coating, wire bar coating, castcoating, roll coating, brush coating, dip coating, spray coating, andcurtain coating. Further, the cross-linking solution can be alsosupplied by injecting into a mold or the like with a prescribed shape.

(B-1) First Cross-linking Step

In the present invention, a cross-linking step is a step of forming afirst carbon nanotube structure that has a network structure composed ofthe plural carbon nanotubes cross-linked with each other through curingof the cross-linking solution after supply.

(A-2) Second Supplying Step

This step is similar to the step A-1, but is different from the step A-1in that the first carbon nanotube structure is supplied with the secondsolution.

(B-2) Second Cross-linking Step

This step is similar to the step B-1. However, the second carbonnanotube structure formed as the result of the second cross-linking stepis combined with the first carbon nanotube structure.

An operation carried out in the cross-linking step is naturallydetermined according to the combination of the functional groups withthe cross-linking agent or the additives for chemically bonding thefunctional groups together. If a combination of thermally curablefunctional groups is employed, the applied solution is heated by variousheaters or the like. If a combination of functional groups that arecured by ultraviolet rays is employed, the applied solution isirradiated with a UV lamp or left under the sun. If a combination ofself-curable functional groups is employed, it is sufficient to let theapplied solution stand still. Leaving the applied solution to standstill is deemed as one of the operations that may be carried out in thecross-linking step of the present invention.

Heat curing (polyesterification through an ester exchange reaction) isconducted for the case of a combination of a carbon nanotube, to whichthe functional group —COOR (where R represents a substituted orunsubstituted hydrocarbon group) is added, and a polyol (among them,glycerin and/or ethylene glycol). Heating causes an ester exchangereaction between —COOR of the esterified carbon nanotube carboxylic acidand R′-OH (where R′ represents a substituted or unsubstitutedhydrocarbon group) of a polyol. As the reaction progressesmultilaterally, the carbon nanotubes are cross-linked until a network ofcarbon nanotubes connected to each other constructs a carbon nanotubestructure layer.

To give an example of conditions preferable for the above combination,the heating temperature is specifically set to preferably 50 to 500° C.,more preferably 150 to 200° C., and the heating period is specificallyset to preferably 1 minute to 10 hours, more preferably 1 hour to 2hours.

In addition, the number of times of combination to be performed is notlimited to two, and may be two or more. Repeated combination enables acarbon nanotube to be structured at a higher density.

The resultant carbon nanotube composite may be utilized as it is.Alternatively, the resultant carbon nanotube composite may be subjectedto patterning through ashing or the like to be used for an electronicdevice. Specifically, the first and second carbon nanotube structuresmay be formed by mainly using single-wall carbon nanotubes exhibitingsemiconductor characteristics. Alternatively, a high-density carbonnanotube structure may be formed by doping impurities, introducingdefects into carbon nanotubes, or the like, which is used as asemiconductor material for a silicon wafer.

Hereinafter, a more specific description of the present invention isgiven by way of an example. However, the present invention is notlimited to the following example.

EXAMPLE

[Carbon Nanotube Composite Structure Using Glycerin-Cross-LinkedMulti-Wall Carbon Nanotube Structure and Single-wall Carbon NanotubeStructure]

(First Addition Step)

30 mg of multi-layer carbon nanotube powder (purity: 90%, averagediameter: 30 nm, average length: 3 μm, available from Science LaboratoryInc.) was added to 20 ml of concentrated nitric acid (a 60 mass %aqueous solution, available from KANTO KAGAKU) for reflux at 120?C for20 hours to synthesize a carbon nanotube carboxylic acid. A reactionscheme of the above is shown in FIG. 3. In FIG. 3, a carbon nanotube(CNT) portion is represented by two parallel lines (same applies forother figures relating to reaction schemes).

The temperature of the solution was returned to room temperature, andthe solution was centrifuged at 5,000 rpm for 15 minutes to separate asupernatant liquid from a precipitate. The recovered precipitate wasdispersed in 10 ml of pure water, and the dispersion liquid wassubjected to centrifugal separation again at 5,000 rpm for 15 minutes toseparate a supernatant liquid from a precipitate (The above processconstitutes one washing operation). This washing operation was repeatedfive more times and lastly a precipitate was recovered.

(Esterification)

30 mg of the carbon nanotube carboxylic acid prepared in the above stepwas added to 25 ml of methanol (available from Wako Pure ChemicalIndustries, Ltd.). Then, 5 ml of concentrated sulfuric acid (98 mass %,available from Wako Pure Chemical Industries, Ltd.) was added to themixture, and the whole was refluxed at 65° C. for 4 hours for methylesterification. The above reaction scheme is shown in FIG. 4.

After the temperature of a solution had been returned to roomtemperature, a precipitate was separated through filtration. Theprecipitate was washed with water and then recovered.

(First Mixing Step)

10 mg of the carbon nanotube carboxylic acid methyl esterified in theabove step was added to 5 ml of glycerin (available from KANTO KAGAKU),and the whole was mixed using an ultrasonic dispersing machine. Further,the mixture was added to 10 ml of methanol, a viscosity modifier.

(First Supplying Step)

About 0.1 ml of the thus obtained coating material was dropped andsupplied onto an SiO₂/Si substrate using a Pasteur pipette.

(First Curing Step)

The substrate to which the coating material of this example had beensupplied as above was heated at 200° C. for 2 hours to startpolymerization by an ester exchange reaction, thereby obtaining aglycerin-cross-linked multi-wall carbon nanotube structure having anetwork structure. A reaction scheme of this is shown in FIG. 5.

(Second Addition Step)

30 mg of single-wall carbon nanotube powder (purity: 90%, averagediameter: 1.2 nm, average length: 1.5 μm, available from ScienceLaboratory Inc.) was added to 20 ml of concentrated nitric acid (a 60mass % aqueous solution, available from KANTO KAGAKU) for reflux at 120°C. for 1.5 hours to synthesize a carbon nanotube carboxylic acid (FIG.3).

The temperature of the solution was returned to room temperature, andthe solution was centrifuged at 5,000 rpm for 15 minutes to separate asupernatant liquid from a precipitate. The recovered precipitate wasdispersed in 10 ml of pure water, and the dispersion liquid wassubjected to centrifugal separation again at 5,000 rpm for 15 minutes toseparate a supernatant liquid from a precipitate. (The above processconstitutes one washing operation.) This washing operation was repeatedfive more times and lastly a precipitate was recovered.

(Esterification)

30 mg of the carbon nanotube carboxylic acid prepared in the above stepwas added to 25 ml of methanol (available from Wako Pure ChemicalIndustries, Ltd.). Then, 5 ml of concentrated sulfuric acid (98 mass %,available from Wako Pure Chemical Industries, Ltd.) was added to themixture, and the whole was refluxed at 65° C. for 4 hours for methylesterification (FIG. 4).

After the temperature of a solution had been returned to roomtemperature, a precipitate was separated through filtration. Theprecipitate was washed with water and then recovered.

(Second Mixing Step)

10 mg of the single-wall carbon nanotube carboxylic acid methylesterified in the above step was added to 5 ml of glycerin (availablefrom KANTO KAGAKU) and the whole was mixed using an ultrasonicdispersing machine. Further, the mixture was added to 10 ml of methanol,a viscosity modifier.

(Second Supplying Step)

About 0.1 mL of a cross-linking solution composed of the single-wallcarbon nanotube carboxylic acid prepared in the above second mixing stepwas dropped onto the glycerin-cross-linked multi-wall carbon nanotubestructure formed in the above first curing step so that theglycerin-cross-linked multi-wall carbon nanotube structure was suppliedwith (immersed in) the cross-linking solution.

(Second Curing Step)

The substrate to which the paint of this example had been applied asdescribed above was heated at 200° C. for 2 hours, followed bypolymerization by an ester exchange reaction to form a network-shapedstructure. FIG. 6 shows an electron micrograph of this structure. Asshown in FIG. 6, the multi-wall carbon nanotubes 5 are cross-linked bythe single-wall carbon nanotubes 6 for combination. Furthermore, thisstructure was evaluated for electric conductivity by a two-probe method.

COMPARATIVE EXAMPLE

An electric conductivity of the multi-wall carbon nanotube structurewhich had been subjected to steps up to the first cross-linking step wasalso measured as a comparative example. The result is shown below.

This Example: 21.4 S/cm

Comparative Example: 10.1 S/cm

The electric conductivity of the carbon nanotube composite structure ofthis example increased to be about twice as high as that of themulti-wall carbon nanotube structure of the comparative example byfilling gaps in a multi-wall carbon nanotube structure with single-wallcarbon nanotubes.

The carbon nanotube composite structure of this example is expected tofind use in various applications such as electric and electronic and/ormolding materials.

As described above, according to the present invention, obtained is thecarbon nanotube structure which is excellent in thermal or electricconductivity or in mechanical characteristics, and is dense in whichbonding between carbon nanotubes is formed with reliability, so thatcharacteristics of a carbon nanotube can be effectively utilized.

1. A carbon nanotube composite structure comprising: a first carbonnanotube structure in which functional groups bonded to plural carbonnanotubes are chemically bonded and mutually cross-linked to construct anetwork structure; and a second carbon nanotube structure in whichfunctional groups bonded to plural carbon nanotubes are chemicallybonded and mutually cross-linked to construct a network structure, thesecond carbon nanotube structure being combined with the networkstructure of the first carbon nanotube structure.
 2. A carbon nanotubecomposite structure according to claim 1, wherein an average diameter ofthe carbon nanotubes constituting the first carbon nanotube structure isdifferent from an average diameter of the carbon nanotubes constitutingthe second carbon nanotube structure.
 3. A carbon nanotube compositestructure according to claim 1, wherein main carbon nanotubesconstituting the first carbon nanotube structure are multi-wall carbonnanotubes, and main carbon nanotubes constituting the second carbonnanotube structure are single-wall carbon nanotubes.
 4. A carbonnanotube composite structure according to claim 1, wherein at least oneof the first carbon nanotube structure and the second carbon nanotubestructure is manufactured by curing a solution containing plural carbonnanotubes to which functional groups are bonded, and by chemicallybonding the plural functional groups bonded to the carbon nanotubes toform cross-linked sites.
 5. A carbon nanotube composite structureaccording to claim 4, wherein the cross-linked sites in at least one ofthe first carbon nanotube structure and the second carbon nanotubestructure are structured by cross-linking the plural functional groupswith a cross-linking agent in the solution, and the cross-linking agentis not self-polymerizable.
 6. A carbon nanotube composite structureaccording to claim 1, wherein each of the cross-linked sites whereplural carbon nanotubes mutually cross-link in at least one of the firstcarbon nanotube structure and the second carbon nanotube structure has achemical structure selected from the group consisting of —COO(CH₂)₂OCO—,—COOCH₂CHOHCH₂OCO—, —COOCH₂CH(OCO—)CH₂OH, and —COOCH₂CH(OCO—)CH₂OCO—. 7.A carbon nanotube composite structure according to claim 4, wherein thecross-linked sites in at least one of the first carbon nanotubestructure and the second carbon nanotube structure are formed throughchemical bonding of the plural functional groups of a same type.
 8. Acarbon nanotube composite structure according to claim 7, wherein areaction forming the chemical bonding is one reaction selected from thegroup consisting of dehydration condensation, a substitution reaction,an addition reaction, and an oxidative reaction.
 9. A carbon nanotubecomposite structure according to claim 1, wherein each of thecross-linked sites where plural carbon nanotubes mutually cross-link inat least one of the first carbon nanotube structure and the secondcarbon nanotube structure is one cross-linked site selected from thegroup consisting of —COOCO—, —O—, —NHCO—, —COO—, —NCH—, —NH—, —S—, —O—,—NHCOO—, and —S—S—.
 10. A method of manufacturing a carbon nanotubecomposite structure, comprising: a first supplying step of supplying asurface of a substrate with a first solution containing plural carbonnanotubes to which functional groups are bonded; a first cross-linkingstep of chemically bonding the plural functional groups to form a firstcarbon nanotube structure in which the plural carbon nanotubes mutuallycross-link to construct a network structure; a second supplying step ofsupplying the first carbon nanotube structure with a second solutioncontaining plural carbon nanotubes to which functional groups arebonded; and a second cross-linking step of chemically bonding the pluralfunctional groups in the second solution to form a second carbonnanotube structure in which the plural carbon nanotubes mutuallycross-link to construct a network structure, the second carbon nanotubestructure being combined with the first carbon nanotube structure.
 11. Amethod of manufacturing a carbon nanotube composite structure accordingto claim 10, wherein an average diameter of the carbon nanotubes in thefirst solution is different from an average diameter of the carbonnanotubes in the second solution.
 12. A method of manufacturing a carbonnanotube composite structure according to claim 10, wherein main carbonnanotubes in the first solution are multi-wall carbon nanotubes, andmain carbon nanotubes in the second solution are single-wall carbonnanotubes.
 13. A method of manufacturing a carbon nanotube compositestructure according to claim 10, wherein: at least one of the firstsolution and the second solution contains a cross-linking agent thatcross-links the plural functional groups together; and the cross-linkingagent is not self-polymerizable.
 14. A method of manufacturing a carbonnanotube composite structure according to claim 13, wherein: each of thefunctional groups in at least one of the first solution and the secondsolution is at least one functional group selected from the groupconsisting of —OH, —COOH, —COOR (where R represents a substituted orunsubstituted hydrocarbon group), —COX (where X represents a halogenatom), —NH₂, and —NCO; and the cross-linking agent is capable ofprompting a cross-linking reaction with the selected functional groups.15. A method of manufacturing a carbon nanotube composite structureaccording to claim 13, wherein: the cross-linking agent is at least onecross-linking agent selected from the group consisting of a polyol, apolyamine, a polycarboxylic acid, a polycarboxylate, a polycarboxylicacid halide, a polycarbodiimide, and a polyisocyanate; and each of thefunctional groups in at least one of the first solution and the secondsolution is capable of prompting a cross-linking reaction with theselected cross-linking agent.
 16. A method of manufacturing a carbonnanotube composite structure according to claim 13, wherein: each of thefunctional groups in at least one of the first solution and the secondsolution is at least one functional group selected from the groupconsisting of —OH, —COOH, —COOR (where R represents a substituted orunsubstituted hydrocarbon group), —COX (where X represents a halogenatom), —NH₂, and —NCO; the cross-linking agent is at least onecross-linking agent selected from the group consisting of a polyol, apolyamine, a polycarboxylic acid, a polycarboxylate, a polycarboxylicacid halide, a polycarbodiimide, and a polyisocyanate; and thefunctional groups and the cross-linking agents are respectively selectedfor a combination capable of prompting a mutual cross-linking reaction.17. A method of manufacturing a carbon nanotube composite structureaccording to claim 14, wherein each of the functional groups is —COOR(where R represents a substituted or unsubstituted hydrocarbon group).18. A method of manufacturing a carbon nanotube composite structureaccording to claim 17, wherein the cross-linking agent is a polyol. 19.A method of manufacturing a carbon nanotube composite structureaccording to claim 17, wherein the cross-linking agent is at least onecross-linking agent selected from the group consisting of glycerin,ethylene glycol, butenediol, hexynediol, hydroquinone, andnaphthalenediol.
 20. A method of manufacturing a carbon nanotubecomposite structure according to claim 10, wherein at least one of thefirst solution and the second solution further contains a solvent.
 21. Amethod of manufacturing a carbon nanotube composite structure accordingto claim 13, wherein the cross-linking agent also functions as asolvent.
 22. A method of manufacturing a carbon nanotube compositestructure according to claim 10, wherein a reaction forming the chemicalbonding in at least one of the first cross-linking step and the secondcross-linking step is a reaction for chemically bonding the pluralfunctional groups of a same type.
 23. A method of manufacturing a carbonnanotube composite structure according to claim 22, wherein at least oneof the first solution and the second solution further contains anadditive that forms the chemical bonding among the plural functionalgroups of a same type.
 24. A method of manufacturing a carbon nanotubecomposite structure according to claim 23, wherein the reaction isdehydration condensation and the additive is a condensation agent.
 25. Amethod of manufacturing a carbon nanotube composite structure accordingto claim 24, wherein each of the functional groups is at least onefunctional group selected from the group consisting of —COOR (where Rrepresents a substituted or unsubstituted hydrocarbon group), —COOH,—COX (where X represents a halogen atom), —OH, —CHO—, and —NH₂.
 26. Amethod of manufacturing a carbon nanotube composite structure accordingto claim 25, wherein each of the functional groups is —COOH.
 27. Amethod of manufacturing a carbon nanotube composite structure accordingto claim 24, wherein the condensation agent is at least one condensationagent selected from the group consisting of sulfuric acid,N-ethyl-N′-(3-dimethylaminopropyl)carbodiimide, and dicyclohexylcarbodiimide.
 28. A method of manufacturing a carbon nanotube compositestructure according to claim 23, wherein the reaction is a substitutionreaction and the additive is a base.
 29. A method of manufacturing acarbon nanotube composite structure according to claim 28, wherein eachof the functional groups is at least one functional group selected fromthe group consisting of —NH₂, —X (where X represents a halogen atom),—SH, —OH, —OSO₂CH₃, and —OSO₂(C₆H₄)CH₃.
 30. A method of manufacturing acarbon nanotube composite structure according to claim 28, wherein thebase is at least one base selected from the group consisting of sodiumhydroxide, potassium hydroxide, pyridine, and sodium ethoxide.
 31. Amethod of manufacturing a carbon nanotube composite structure accordingto claim 22, wherein the reaction is an addition reaction.
 32. A methodof manufacturing a carbon nanotube composite structure according toclaim 31, wherein each of the functional groups is at least onefunctional group selected from the group consisting of —OH and —NCO. 33.A method of manufacturing a carbon nanotube composite structureaccording to claim 22, wherein the reaction is an oxidative reaction.34. A method of manufacturing a carbon nanotube composite structureaccording to claim 33, wherein each of the functional groups is —SH. 35.A method of manufacturing a carbon nanotube composite structureaccording to claim 33, wherein the solution further contains anoxidative reaction accelerator.
 36. A method of manufacturing a carbonnanotube composite structure according to claim 35, wherein theoxidative reaction accelerator is iodine.